The first flight, in June 2002, ended in disaster when the vehicle had to be destroyed in mid-air due to "stabilization problem with the booster rocket's directional fins".

The Pegasus booster rocket in question is required to accelerate the X-43A to a sufficient speed for the scramjet to operate. After that, it's all pretty simple: the scramjet ducts air directly from the atmosphere, mixing it with hydrogen before combustion. The forward speed of the vehicle provides compression, thereby eliminating the need for conventional jet engine turbines. The speed of the airflow through the engine remains supersonic throughout.

The test will take place over the Naval Air Warfare Center Weapons Division Sea Range off the southern coast of California. The X-43A and booster will be dropped from a B-52 at around 40,000ft. The booster will carry it to 100,000ft at which point the X-43A will separate and then attempt a ten-second scramjet-powered flight.

NASA has high hopes for scramjet technology, which it is developing as part of its Hyper-X program. The agency eventually hopes to develop hypersonic passenger transports and other spin-offs.

One group of scientists who will certainly be watching Saturday's test with interest is the Australian team which claimed to have already successfully tested a scramjet back in July 2002. ®